According to my 69 FSM the driveshaft for a V8 with 4 speed and 8 & 3/4 is 51.53 inches measured from the "centerline of front bearing bore to centerline of rear bearing bores" (I'd say the definition of "bearing bore" is where the u-joints are installed). Since the 69 Dart has the same wheelbase as a 66 Dart, I'd venture a guess and say that's the measurement you need.